This invention relates to electrostatographic reproduction machines, and more particularly to a liquid electrophotographic reproduction machine using elevated temperature heated carrier liquid for improved developability.
Liquid electrophotographic reproduction machines are well known, and generally each include a development system that utilizes a liquid developer material typically having about 2 percent by weight of fine solid particulate toner material dispersed in a liquid carrier. The liquid carrier is typically a hydrocarbon. In the electrophotographic process of such a machine, a latent image formed on an image bearing member or photoreceptor is developed with the liquid developer material. The developed image on the photoreceptor typically contains about 12 percent by weight of particulate toner in liquid hydrocarbon carrier. To improve the quality of transfer of the developed image to a receiver, the image is conditioned so as to increase the percent solids of the liquid developer forming the image to about 25 percent. Such conditioning is achieved by removing excess hydrocarbon liquid from the developed liquid image. However, such removal must be carried out in a manner that results in minimum degradation of the toner particles forming the liquid image. The conditioned image is then subsequently transferred to a receiver which may be an intermediate transfer belt and then to a recording or copy sheet for fusing to form a hard copy.
Liquid electrophotographic reproduction machines as such can produce single color images or multicolor images on such a recording or copy sheet. The quality or acceptability of a color copy produced as such is ordinarily a function on how the human eye and mind receives and perceives the colors of the original and compares it to the colors of the copy. The human eye has three color receptors that sense red light, green light, and blue light. These colors are known as the three primary colors of light. These colors can be reproduced by one of two methods, additive color mixing and subtractive color mixing, depending on the way the colored object emits or reflects light.
In the method of additive color mixing, light of the three primary colors is projected onto a white screen and mixed together to create various colors. A well known exemplary device that uses the additive color method is the color television. In the subtractive color method, colors are created from the three colors yellow, magenta and cyan, that are complementary to the three primary colors. The method involves progressively subtracting light from white light. Examples of subtractive color mixing are color photography and color reproduction. Also, it has been found that electrophotographic reproduction machines are capable of building up a full subtractive color image from cyan, magenta, yellow and black. They can produce a subtractive color image by one of three methods.
One method is to transfer the developed image of each color on an intermediary, such as a belt or drum, then transferring all the images superimposed on each other on a sheet of copy paper.
A second method involves developing and transferring an image onto a sheet of copy paper, then superimposing a second and subsequent images onto the same sheet of copy paper. Typically an image processing system using this method can produce a first color image by developing that color image on a photoconductive surface, transferring the image onto a sheet of copy paper, and then similarly and sequentially producing and superimposing a second, and subsequent images onto the same sheet of copy paper.
A third method utilizes what is referred to as a Recharge, Expose, and Develop or REaD process. In this process, the light reflected from the original is first converted into an electrical signal by a raster input scanner (RIS), subjected to image processing, then reconverted into a light, pixel by pixel, by a raster output scanner (ROS) which exposes the charged photoconductive surface to record a latent image thereon corresponding to the substractive color of one of the colors of the appropriately colored toner particles at a first development station. The photoconductive surface with the developed image thereon is recharged and re-exposed to record the latent image thereon corresponding to the subtractive primary of another color of the original. This latent image is developed with appropriately colored toner. This process (READ) is repeated until all the different color toner layers are deposited in superimposed registration with one another on the photoconductive surface. The multi-layered toner image is transferred from the photoconductive surface to a sheet of copy paper. Thereafter, the toner image is fused to the sheet of copy paper to form a color copy of the original.
Liquid developers when utilized in machines making single color (black and white) images or multicolor images according to any of the above methods, have many advantages over dry developer materials or toners. For example, liquid developers often result in images of higher quality than images formed with dry toners. Liquid toner particles can usually be made relatively very small without resulting in problems often associated with small particle powder toners, problems such as machine dirt which can adversely affect process reliability. Development with liquid developers in full color imaging processes also has many advantages, such as a texturally attractive print because there is substantially no height buildup, whereas full color images developed with dry toners often exhibit height build-up of the image where color areas overlap. Further, full color prints made with liquid developers can be made to a uniformly glossy or a uniformly matte finish, whereas uniformity of finish is difficult to achieve with powder toners because of variations in the toner pile height, the need for thermal fusion, and the like.
In Liquid electrophotographic reproduction machines as disclosed for example in U.S. Pat. No. 5,028,964 the conventional practice and strategy are for running the development process of each such machine at near room temperature, and for heating the developed images preferably on the intermediate transfer member, after initial transfer to such intermediate member. As such, the intermediate member, such as a belt, must thereafter be actively cooled before the next image is transferred to it from the photoreceptor.
In such machines, it is also desirable to use low vapor pressure carrier liquids, such as mineral oils, in which to disperse toner particles. Unfortunately however, this desire ordinarily when achieved, is offset by a penalty of relatively higher carrier liquid viscosity, and hence a penalty of reduced toner mobility within the carrier liquid. Furthermore, use of such mineral oil-based inks or liquid toners may require the use of relatively larger or additional rollers in the development subsystem. The use of such rollers would understandably increase significantly the cost and size of the development subsystem, and may even require larger photoreceptor belts, and hence larger and more costly machines overall.
There is therefore a need for a liquid electrophotographic reproduction machine which uses a very low vapor pressure mineral oil for the carrier fluid, and yields improved developability without significant increased cost.